High Limit Switch, also known a “Fan Limit Switch” or simply “Limit Switch” plays an important role from safety point of view. The main function of a Limit Switch is to sense the temperature in the air plenum A plenum space is a part of a building that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure.Plenum space
What does a limit switch do in a furnace?
The furnace limit switch is a simple logic circuit that controls your home’s maximum level of heating. Its primary purpose is to shut down the furnace if overheating is detected in your interiors. Additionally, the limit switch also turns the blower fan on and off during regular heating cycles to prevent overheating in the first place.
How to replace the fan limit switch in a furnace?
Where is the Furnace Fan Limit Switch Located?
- Turn off the furnace. Switch the furnace off at the circuit breaker. Then switch off the power at the furnace too.
- Remove the access panel. Most panels come off without removing any bolts or screws. ...
- Locate the fan limit switch. The switch is mounted on the furnace air plenum and is easily recognizable due to the wiring. ...
What does a Furnace Fan limit switch do?
To put it very simply, the furnace fan limit switch is a control that tells your furnace's fan when to turn on and off. So, when the furnace isn't on, it tells the blower not to operate (and send cold air into your home) and when the furnace is on, it tells the blower to turn on and start circulating the warm air.
Where is the Furnace Fan limit switch located?
Your fan limit switch is located inside the furnace panel cover, usually near the upper right corner. The fan limit switch is responsible for turning the blower on and off at the appropriate times.
What causes a high limit switch to trip?
The high limit switch has a flame sensor rod that detects air temperatures in the plenum of the furnace. If it is dirty, it cannot properly detect temperatures, which is one reason why the furnace high limit switch trips.
How do you reset a high limit switch on a furnace?
If your limit switches trips repeatedly, it can enter a lockdown mode. That is, it shuts down completely. You can try resetting the furnace limit switch yourself if you have a newer model by turning off the furnace for 30 seconds and then turning it back on.
How do you know if a limit switch is bad?
The most common sign of a malfunctioning limit switch is the continuous operation of your furnace's blower. Just as the limit switch won't allow the fan to turn on until the warm air has reached the right temperature, the limit switch also shuts down when the air cools to a certain temperature.
How many high limit switches are on a furnace?
The mounting plate usually contains two or more terminals that receive control wires for the blower fan and the furnace's gas valve. It's possible that your furnace will have two or even three limit switches that check temperatures at different parts of the furnace.
How much does it cost to replace a limit switch on a furnace?
The cost to replace a limit switch on a furnace is $100 to $375 on average. The fan-limit switch has sensors that shut off the blower motor when it overheats.
Can you run a furnace without a limit switch?
A furnace limit switch is a component of a forced-air furnace that is responsible for turning on the furnace blower once the desired heat has been met inside the furnace. Without a functioning furnace limit switch, a furnace would not be able to regulate the temperature in conjunction with a thermostat.
How long do limit switches last?
In evaluating the specification, you will find that the AutomationDirect limit switch has an astounding mechanical life of 30 million operations, while the electrical life is an incredible 5 million operations.
How do you test a limit switch on a gas furnace?
1:143:34Furnace - Limit Switch | Repair and Replace - YouTubeYouTubeStart of suggested clipEnd of suggested clipTake the multimeter and set it to the lowest ohms of resistance. Now touch the probes to eachMoreTake the multimeter and set it to the lowest ohms of resistance. Now touch the probes to each terminal if the switch has continuity then the multimeter will show zero ohms.
What causes a furnace to not provide enough heat?
Check Your Vents & Ducts. The harder your furnace has to work to push air through the ventilation system, the longer it takes to bring the heat up. Dirt, dust and debris cause your furnace to work harder than necessary. Keep vents clear and ductwork clean.
Can you clean a high limit switch?
Remove the high limit switch by removing the two screws that hold it to the plenum. If the flame sensor rod is dirty, you can try cleaning it gently with steel wool or sandpaper, or replace the part with an identical new part.
Why does my furnace keep tripping the reset button?
If your furnace reset button keeps tripping, it's most likely that your heater is overheating. This may indicate a need for heating system repair work that needs to be addressed.
Where is the furnace limit switch located?
The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off.
Where is furnace reset button?
All furnaces are equipped with a reset button. They are generally red or yellow and are located inside the blower compartment. The reset button is a safety feature that will shut down the furnace when a problem is detected, often a hot or overheating furnace, before it trips the circuit breaker.
How do you jump a limit switch?
0:212:48Jumping Out Limit Switches - YouTubeYouTubeStart of suggested clipEnd of suggested clipAnd keeping in mind when you do this you'll have to stop the gate opener in both directions byMoreAnd keeping in mind when you do this you'll have to stop the gate opener in both directions by hitting button one on the gate on the control board we'll go ahead and put these jumper wires in.
How do you test a limit switch on a gas furnace?
1:143:34Furnace - Limit Switch | Repair and Replace - YouTubeYouTubeStart of suggested clipEnd of suggested clipTake the multimeter and set it to the lowest ohms of resistance. Now touch the probes to eachMoreTake the multimeter and set it to the lowest ohms of resistance. Now touch the probes to each terminal if the switch has continuity then the multimeter will show zero ohms.
Is a high limit switch normally open or closed?
The limit switch is a normally closed switch in series with the sail switch and the thermostat. Being temperature actuated, it is designed to "snap open" when an over temperature condition occurs. Thus the switch is normally opening under full load conditions and that causes arcing of the contacts.
Why is a furnace limit switch important?
That’s why the furnace limit switch is so important. It’s also why a furnace starts shutting down unexpectedly if the switch starts to fail. For safety reasons, heating systems are designed such that without a properly functioning limit switch, they will not operate.
What is a limit switch?
The limit switch—sometimes called a “high-limit switch,” which gives more of an indication of its function—protects your furnace and your home or business by automatically bringing the temperature of your furnace down if it gets too high .
What happens when the furnace temperature is back in normal range?
When the furnace temperature is back in the normal operating range, the limit switch allows the burners to activate again.
How to tell if a limit switch is failing?
First, if the furnace blowers don’t seem to shut off when they should, that ’s an indicator. And second, if the air coming out of vents is cool rather than warm, that’s a sign, as well.
Why does my limit switch trip?
For example, the limit switch gets information from a temperature sensor. If that sensor gets dirty, which sometimes happens , that can cause the limit switch to trip.
Can a furnace get too hot?
Not only would allowing a furnace to get too hot potentially damage the equipment, but it also would pose a threat to people in the home or business. First, there would be the risk of fire. However, a damaged furnace could also allow dangerous carbon monoxide to escape.
Why is a furnace limit switch important?
The furnace limit switch is an essential component because it prevents your furnace from becoming too warm throughout standard operation. This is a safety feature that is designed to prevent damage to the internal components of your furnace and keep your family safe. In addition, the furnace fan limit switch is in charge if controlling ...
What is a furnace fan limit switch?
Furnace fan limit switches also include a maximum temperature function that will disable the standard heating process if the furnace overheats. Lastly, the furnace fan limit switch may also be accompanied by a Manual On switch that allows you to keep the fan activated continuously throughout the day in your home to improve air flow.
Why is my furnace deactivating?
If your furnace is continuously activating and deactivating throughout the day, this may also be a sign of a blower motor issue. An overheated blower assembly will trip the furnace limit switch and cause your furnace to deactivate prior to the end of the standard heating cycle. Other common issues that may affect the furnace fan limit switch ...
How does a furnace produce heat?
The burners are designed to heat the air produced by the air plenum in your furnace. Each burner works in conjunction with the furnace limit switch to ensure the air from the plenum is at least 20 degrees above the room temperature.
Does the furnace blower remain in session?
However, the blower assembly will remain in session until the furnace limit switch confirms the internal furnace temperature is at a safe level. Ways to Decrease Your Air Conditioning Bills.
Can a clogged furnace filter cause a furnace to overheat?
Lastly, we always recommend checking the filter in your furnace to confirm it isn’t clogged. A clogged air filter may cause the plenum in your furnace to reach an unsafe temperature. Once the plenum overheats, the furnace limit switch will turn off the furnace to keep you safe. To receive help with a furnace limit switch repair, ...
Why do furnaces have high limit switches?
The furnace high limit switch helps to increase furnace efficiency. As the home cools, the thermostat sends a signal back to the furnace to kick on, initiating the cycle once more.
What is a furnace limit switch?
A furnace limit switch is a component of a forced-air furnace that is responsible for turning on the furnace blower once the desired heat has been met inside the furnace. Without a functioning furnace limit switch, a furnace would not be able to regulate the temperature in conjunction with a thermostat.
How Does A Furnace High Limit Switch Regulate Temperature?
When the heat is turned on or up, a signal is sent from the thermostat to the furnace, turning on the unit. Once the furnace heats up , the furnace limit switch closes, connecting the circuit and turning on the blowers. Once the home or business reaches the desired temperature, the furnace burners will shut off—but the blower will keep running until the temperature inside the furnace decreases to the lower limit (the coolest air temperature permitted as set by the thermostat).
How Do I Know If My Furnace Limit Switch Is Broken?
Most homeowners haven’t heard of a furnace limit switch because, if it works properly, you wouldn’t know it was there . Homeowners should have their furnace limit switch inspected if:
Why do you need a limit switch on a furnace?
A furnace limit switch isn’t just important to the cycling of your furnace it also will shut off the furnace blowers and burners if the unit overheats. An overheated furnace is a safety risk that can lead to serious furnace damage. It's important for homeowners to know that furnace overheating most often occurs when the blower fan malfunctions ...
Where is the furnace pressure switch located?
Equally important, a furnace pressure switch, located inside a furnace motor compartment, is designed to detect negative pressure inside the unit and, when detected, shut the furnace down.
Can I Repair My Own Furnace High Limit Switch?
Yes, but it is not recommended. While this task may seem simple to an experienced HVAC technician or electrician, it is often daunting to a homeowner. Improperly completed furnace repairs may leave your property vulnerable to fire, carbon monoxide leaks, and more.
Why use a high limit switch?
The high limit switch helps to protect the heat exchanger from getting cracked from excessive temperatures.
Why does my furnace limit switch trip?
With less airflow going through your furnace, the heat exchanger excessively heats up and causes the limit switch to trip .
How to remove high limit switch from plenum?
Remove the high limit switch from the plenum by unscrewing it and taking out the wire connectors. Note: Do not touch the sensor. Hold it by the base instead. Oil from your fingers can create a hotspot that will cause the sensor to crack.
Why does my furnace trip out?
The high limit switch in a furnace can sometimes get dirty. When the limit switch is dirty, it will have trouble working properly, causing your furnace to trip out excessively.
How to clean a limit switch sensor?
Clean the limit switch sensor using fine-grit sandpaper or a scouring pad. Avoid steel wool since that will scratch the sensor surface.
What to do if you are not comfortable working on your furnace?
Note: If you’re not comfortable working on your furnace, then call an HVAC professional to check it out instead.
What does the plenum do when the temperature is too high?
It senses the temperature in the air plenum– and shuts off the burner if the temperature gets too high.
Where is the high limit switch on a gas furnace?
The high limit switch on a gas furnace is located at or near the burner with two wires coming out of it and leading back into the panel box. It will be mounted either vertically or horizontally in relation to how you’re looking at it. The new one should come with instructions from its manufacturer to help you with the replacement.
What does a high limit switch do?
The response is to shut down the system as a safety measure.
What causes the furnace high limit switch to keep tripping?
Oftentimes, tripping is because of one of three reasons: an inefficient furnace filter, faulty wiring or installation, or not enough airflow coming into the furnace.
Why does my furnace cycle on and off?
90% of the time, a gas furnace cycling on and off is a sign the flame sensor rod is dirty and needs cleaning. If you’re able to open the access panel, you can clean the sensor yourself, but if you don’t know how to access it, I’d suggest you get a technician to help you with the process.
Why do limit switches wear out?
Limit switches wear out over time and can even become defective especially when other parts of the gas furnace are faulty too. Defects usually come from overuse – where the HLS trips too often – and lack of maintenance.
Why is my furnace overheating?
An overheating furnace is sometimes caused by a dirty air filter. In turn, restricted airflow leads to erratic tripping and opening of the high limit switch in your furnace.
What does it mean when a furnace is in lockout mode?
It means that the furnace is overheating to a severe level that can easily start a fire in your home.
What is a furnace fan limit switch, and what is its function?
It is a small device that consists of a long temperature-sensing probe placed inside the furnace and a metal switch mounting plate located outside . It is a control circuit whose prime function is to turn the blower fan on or off, based on the pre-temperature settings during the regular furnace operation. It also prevents any damage to the furnace from overheating by shutting it down. It monitors the temperature in the combustion chamber to control the unit’s operation.
Why turn off the burner on a furnace?
Turn off the burner if the furnace gets too hot: A dirty and clogged filter or malfunctioning blower fan can restrict the smooth airflow through the heat exchanger. It overheats the unit leading to cracks in the heat exchanger pipe, resulting in carbon monoxide gas leaks. When the fan limit switch senses that the supply air temperature is very high and the heat exchanger is overheating, it trips to shut off the gas valve to stop the gas supply to the burner and halt the heating process.
How to turn off blower fan?
Turn off the blower fan: Once the thermostat detects that circulating air has reached the desired temperature setting, the furnace burner turns off. The blower fan continues to run briefly till the supply air reaches the lower temperature limit set on the thermostat. When it happens, the limit switch shuts off the blower fan motor till the next cycle.
What is a furnace?
A furnace is a vital part of the central HVAC systems most commonly used to heat residential and commercial buildings. It has several components that need to work correctly for the furnace to generate heat.
Can a furnace limit switch wear out?
As a furnace gets old, it is not uncommon for the limit switch to wear out or develop issues. A blower fan running continuously or a non-operational furnace indicates a problem with the limit switch. Call an HVAC company to inspect, repair, or replace a bad limit switch.
What is the responsibility of a high limit switch?
Fan motor. The first and most important responsibility of the high limit switch is to turn the fan motor on and off. This happens in every single cycle of normal operation for your furnace.
Where is the high limit switch located?
The switch is then wired into the blower fan motor as well as the gas valve. You can typically find the switch just past where the heat exchanger is located .
What temperature does a high limit furnace go into lockout mode?
When that happens, your switch may start tripping at lower temperatures than normal. Normally, a high limit switch might start to trip around 155-degrees Fahrenheit. Instead, you may notice that it trips at around 130 degrees. This is what can lead to the furnace going into lockout mode over time even without overheating.
How to tell if a high limit switch is bad?
Signs That the High Limit Switch is Bad 1 Not signaling the blower. Perhaps the number one sign that your high limit switch is going bad is that it is failing to signal the blower fan, primarily when turning off. If the fan keeps on running and you’ve made sure that the thermostat is set to AUTO instead of ON, then it could mean that your high limit switch is going bad. 2 Lower temperatures. Time can weaken your high limit switch, wearing it down. When that happens, your switch may start tripping at lower temperatures than normal. Normally, a high limit switch might start to trip around 155-degrees Fahrenheit. Instead, you may notice that it trips at around 130 degrees. This is what can lead to the furnace going into lockout mode over time even without overheating. 3 Replacing your switch. If either of these cases become a regular occurrence, the high limit switch will need replacing. The tricky part is determining whether or not the switch is actually bad or not. Overheating can still occur, leading to the switch tripping far more regularly.
Why does my furnace keep tripping?
The last thing that you want is for your furnace to continue failing, leading to bigger and more expensive repairs. One such issue that you may be experiencing is that the high limit switch keeps tripping. When the high limit switch keeps tripping, there are two likely causes. The first is overheating. When the unit overheats, one of the results is ...
Why does my high limit switch keep tripping?
When the high limit switch keeps tripping, there are two likely causes. The first is overheating. When the unit overheats, one of the results is that the high limit switch keeps tripping. The other cause of this problem is that the switch itself is going bad.
Why is it important to protect a high limit switch?
Protecting your high limit switch is important due to the safety role that it plays. The more that it gets tripped due to overheating, the more that it wears down. Thankfully, there is a relatively easy solution when it comes to protecting your high limit switch.
